Voltage instability is closely related to the notion of maximum loadability of a transmission network. In present-day power systems, this may take place as a precursor to the traditional frequency instability problem (see Proceedings of Bulk Power System Voltage Phenomena-III: "Voltage Stability, Security and Control," Davos, Switzerland, August 1994; and K. Vu, et al., "Voltage Instability: Mechanisms and Control Strategies," Proc. of IEEE, Special Issue on Nonlinear Phenomena in Power Systems, pp. 1442-1455, November 1995). It is critical for the utility company to track how close the transmission system is to its maximum loading. If the loading is high enough, actions have to be taken to relieve the transmission system.
A problem associated with tracking the maximum loading of the transmission system is that such maximum loading is not a fixed quantity, but rather depends on the network topology, generation and load patterns, and the availability of VAR resources. All of these factors can vary with time due to scheduled maintenance, unexpected disturbances, etc.
Despite the fact that voltage instability is a system problem, there is still a need for relays that process only local measurements. These relays are to be counted upon when other controls cannot mitigate the situation; they also form the fall-back position for any global protection scheme when communication channels fail. Controls that use only local data provide an attractive approach because they are low cost and simple to build. The most common form is to shed load based on voltage level--under-voltage load shedding. This scheme has been attempted on the Pacific Northwest system, as reported by C. W. Taylor, "Power System Voltage Stability," McGraw Hill, 1994. However, for many other systems, the difficulty with choosing the setpoint poses a challenge. In fact, voltage is often a poor indicator of instability, and a fixed setpoint may result in unnecessary shedding or failure to recognize an instability. Some systems may ride through voltages much below the setpoint of the relay but, for others, the voltage can appear normal even though the grid is on the verge of instability. The idea of using an adjustable voltage setpoint has been known, as reported in IEEE Power Systems Relaying, Committee, Working Group K12, Voltage Collapse Mitigation, 1995.
The true goal of a local relay should be to determine whether the load connected to the substation is excessive. A fundamental issue here is whether the transmission system's strength can be "sensed" from local measurements. It has been well known that conventional, local quantities such as voltage level and reactive reserve are poor indicators of voltage instability, and therefore advanced methods are needed. For example, the use of artificial intelligence on local measurements is disclosed in K. Yabe, et al., "Conceptual Designs of AI-based Systems for Local Prediction of Voltage Collapse," IEEE 95 WM 181-8 PWRS. The idea is to simulate a range of system conditions to generate patterns in local observations. In the real environment, true measurements are then compared against known patterns, from which the proximity to collapse is inferred.
The Voltage Instability Predictor
As mentioned, the present application is a continuation-in-part of the '983 application, which discloses a Voltage Instability Predictor, or VIP, that estimates the strength/weakness of a transmission system based on local voltage and current measurements, and compares that with the local demand. The closer the local demand is to the estimated transmission capacity, the more imminent is the voltage instability. This information is used for load shedding as well as other applications.
The operation of the VIP may be summarized as follows: Current and voltage waveforms are measured at the bus, and then current and voltage phasors are derived. Based on the phasors, an apparent impedance associated with the load and a Thevenin impedance associated with the source are determined. The Thevenin impedance and apparent impedances are then compared. The VIP decides whether to initiate a prescribed action, such as load shedding and/or controlling on-load tap-changing (OLTC) transformers, based on the relationship of the apparent impedance to the Thevenin impedance. Further details of the VIP are provided below.